a compound of silicon, oxygen, and hydrogen, regarded as the parent substance from which is derived a large family-the silicates-of minerals, salts, and esters. The acid itself, having the formula Si(OH)4, can be prepared only as an unstable solution in water; its molecules readily condense with one another to form water and polymeric chains, rings, sheets, or three-dimensional networks that constitute the structural units of silica gel (q.v.) and many minerals that have very low solubility in water.